Sound-Level Meter

a device for measuring loudness (of noise) objectively. A sound-level meter consists of a nondirectional microphone, an amplifier, corrective filters, a detector, and an indicating meter.

The overall circuit design is such that the meter’s characteristics approximate the properties of the human ear. The sensitivity of the ear is a function of the frequency of a sound, and this function varies with the intensity of the noise (or sound) being measured. Therefore, there are three sets of filters in a sound-level meter that provide the necessary contour of the frequency response at three loudness levels. Scale A conforms to the response for a low loudness level of —40 phons (it is used in the range 20 to 55 phons); scale B, for a medium loudness level of ~70 phons (55 to 85 phons); and scale C, for a high loudness level, 85 to 140 phons. The response at the high loudness level is uniform over a frequency band of 30 to 8,000 hertz.

Scale A is also used to measure the loudness level in decibels with the notation A, that is, dB(A), at any loudness level. The value of a sound level in dB(A) is used when rating loudness in industry, dwellings, and transportation facilities. The filters are switched manually according to the loudness level of the sound (or noise) being measured. The rectified signal from the square-law detector is averaged over a time interval corresponding to the time constant of the ear—approximately 50 to 60 milliseconds (the time interval in which two separate acoustic signals are perceived as a unified signal by the ear because of its time lag). The scale on the output meter is calibrated in decibels relative to a root-mean-square sound-pressure level of 2 × 10^–5 Pa on one of the three aforementioned scales—A, B, or C.

A modern sound-level meter is a compact, portable instrument powered by self-contained dry batteries. The microphone, electronic circuitry, and indicating meter should be extremely stable with respect to variations in the temperature, humidity, and barometric pressure, as well as invariant with respect to time.